1. Field of the Invention
The present invention relates to a plant for transferring molten glass from a furnace to a station for forming objects, in particular hollow glass objects.
2. The Prior Art
In the soda-lime glass industry, two types of quality can be distinguished at present: on the one hand, the quality for the small bottle and domestic glassware industry and, on the other hand, the quality for the bottle industry.
The quality requirements of the glass for the small bottle and domestic glassware industry are very high, which is why the working tanks and the transfer channels are always equipped with high-quality refractory products, normally electrocast aluminous products. These products, in contact with glass, produce only a negligible quantity of crystallized, vitreous and/or gaseous inclusions.
The quality requirements of the glass for the bottle industry are less high, and it is possible to accept minor defects of the "small cord", "scratch", "bubble" and "seed" type, which can often be made less serious by the dark colour of the glass. These defects are in most cases related to the quality of the refractories used to form the transfer channel which takes the glass from the furnace to the forming plant.
In the state of the art, use is made of alumino-silicate or mullite agglomerated refractories, the cost of which is, obviously, less than that of the electrocast aluminous products. In view of the increase in the performance and productivity levels of the furnaces, the corrosion resistance of the refractories used to date is tending to become inadequate and, moreover, it is obvious that an improvement in quality is always desirable provided it does not result in too great a rise in cost prices. For this reason, there is a tendency at present to replace the alumino-silicate or mullite agglomerated refractories by refractories of the "AZS" type, that is to say alumina-zirconia-silica refractories, which are agglomerated or electro-cast. These products have a better corrosion resistance than the refractories currently used, but, in contact with glass, a highly viscous "alumina-zirconia" phase is formed, whose diffusion and dissolution rate in glass becomes virtually zero in the downstream part of the transfer plant where temperatures below 1250.degree. C. prevail. This phase, which is very low in volume, tends to flow, by density and convection, from the walls of the transfer channel to the bottom thereof and to move from there towards the feeder orifice or orifices.
After a certain operating time, a stream of the "alumina-zirconia" phase reaches the feeder orifices, resulting in a defect which is commonly called "cat scratches" in the finished article. These are a set of very fine parallel vitreous cords which are situated on the outer surface of the glass and have a cross-section no greater than 10 to 20 micrometres. Generally, these defects are visible only when the object is subjected to intense light.
The increase in the quality level of certain articles of the bottle industry, however, no longer permits acceptance of this "defect" and various means have been proposed, or are employed, to eliminate it. Among the means employed, "bubblers" may be mentioned, these being placed at the inlet of the transfer channel and consisting of a device which injects gas bubbles into the lower part of the glass flow, thereby generating an upward current of glass. These bubblers can be installed only at the upstream part of the channel, or in the furnace itself, since the viscosity of the glass in the downstream part would lead to the presence of bubbles in the product obtained. Other means consist of mechanical agitators, or rotating rotors, which can be situated in the downstream part of the channel, or in the feeder bowl, these agitators and rotors permitting dispersion, burbling or stretching of the alumina-zirconia stream, thus reducing the resulting defects to a size such that they are no longer problematical. The installation of these devices, which operate under difficult conditions and in a very viscous medium, is however tricky. Moreover, they are, obviously, relatively expensive.
In addition, U.S. Pat. No. 3,127,262 discloses a process and an installation for refining glass, which are characterized by the use of a sill or barrier arranged at the bottom of the glass-transfer channel, just before the bowl in which the orifice for feeding the molten glass is provided, combined with side drains provided either on the sill itself or in the side walls of the bowl, in the immediate vicinity of the sill and approximately level with the top of the sill. According to the patent mentioned, the aim of the sill is to cause the viscous lower layer of glass, which is heavy and impure, to rise and it is eliminated via the side drains. The purpose of the sill or barrier used is not therefore to retain heavy impurities, but merely to deflect the heavy and impure current of glass, which flows upwards and in the direction of the side drains. This arrangement has not however, as far as the Applicant knows, been developed in practice, no doubt owing to poor efficiency.